With under 24-hours to go until polls close in Jeremy Corbyn’s leadership contest with Owen Smith, the shadow chancellor also condemned the “disgraceful” way the election had been handled by the Labour Party’s ruling National Executive Committee (NEC).

McDonnell, a close ally of Corbyn, was speaking to campaign activists at a phone bank in Clapham, south London, on Tuesday evening.

In an attempt to manage expectations, he warned Labour MPs who want rid of Corbyn that a win for the Labour leader on Saturday was a win - no matter what his margin of victory.

“I think it’s going to be really tough to get the 59.5% that we got last time around because of the numbers that have been prevented from voting,” he said. “Some will argue if we dip below the 59.9% that somehow Jeremy’s mandate has lessened. If we win, no matter what, his mandate is still in place.”

Just four days ago, McDonell said Corbyn needed to “win decisively - increasing the mandate he won last year” in order to bring “stability to our party”.

McDonnell was speaking as Labour’s ruling National Executive Committee (NEC) decided to postpone a decision on whether to grant MPs the right to elect the shadow cabinet.

Watson, Labour’s deputy leader, has proposed giving MPs the right to choose the party’s frontbench as a way of “bringing the band back together” following a bitter leadership contest.

However Corbyn has lent his support to the idea of allowing Labour members to choose some of the shadow cabinet - a move that would likely ensure he can keep key allies in post.

Watson has also proposed the NEC scrap the current leadership rules, including the registered supporters category which was seen as key to Corbyn’s 2015 victory, and return to an electoral college system.

However McDonnell savaged the idea:

“One thing about our party now is it’s based on one person one vote. I don’t want to go back to some of the proposals that have come forward of an electoral college.

“Because what that would mean is the Parliamentary Labour Party gets a third of the votes. It will mean for every MP, their vote will mean 3,000 or 4,00 of ours, and that is unacceptable.

“We want establish a good working relationship with the PLP, as we had before the coup was launched, but you can’t have a situation where those people who are selected and elected on a basis of a membership selecting and campaigning for them, you can’t have situation where they then hold the balance of power on the basis of having one MP with 3,000 votes - it unacceptable, it’s not democratic.”

McDonnell also claimed, despite the widespread expectation that Corbyn will cruise to victory, the campaign had “problems in terms of what will happen on Saturday” due to the number of people who back the Labour leader who had been blocked from voting by party HQ.

“The ruling out of 130,000 members was a disgrace. An absolute disgrace. And the way it was done was even more disgraceful,” he said.

“We lost, I think, a large group of people who are Jeremy Corbyn supporters, never again should we allow that to happen.”

If we win, no matter what, his mandate is still in place
John McDonnell

McDonnell said the Labour Party had banned a “large number” of people from voting in the election for the “bizzarest of reasons”. He has previously accused the party of trying to rig the contest in favour of Owen Smith.

The shadow chancellor said the party’s decision to purge people who had used the word “traitor” made no sense, as it features in the party anthem - The Red Flag. “We are all out on that basis,” he said.

Handout via Getty Images

The overwhelming majority of Labour MPs do not support Corbyn. But McDonnell told them to act in a “comradely” way after the leadership result.

“If there are issues they are concerned about, if it is about the leadership, if it is about policies, if it is about the style of operation, if it is about competence, work with us. Let’s be honest, if there is a criticism about how we have operated over the last 12 months, tell us straightforwardly and we can resolve this,” he said.

And he claimed Labour had been “winning elections” until the majority of the shadow cabinet quit and triggered a leadership contest in June. “In that last raft of half a dozen polls we were level and we were going ahead of the Tories, we thought we were laying the foundations for electoral victory,” he said.

Corbyn has held over 35 campaign rallies during the leadership contest, and McDonnell said they had been “enormous” and had spoken to over 100,000 people. “In Ramsgate, 2,000 people on the beach, the only thing that shut him up was the tide coming in,” he said of his ally.

The result of the Labour leadership contest will be announced at midday on Saturday at the start of the party’s annual conference in Liverpool.